DESCRIPTION: The master central pacemaker for circadian rhythmicity appears to be situated in the suprachiasmatic nucleus (SCN). The SCN is entrained to variations in light intensity via signals from retina, but exactly what information the retina supplies to the SCN is unknown. Neither the ganglion cell type(s) that project to SCN, nor their linking circuits to the photoreceptor, nor the photoreceptor type(s) themselves have been identified. The same axons that go to the SCN appear to reach the ventral lateral geniculate making this an additional site of interest for analysis of circadian rhythmicity. Behavioral studies in a variety of mammals suggest an action spectrum peaking at 500 nm (implicating rods) and a threshold intensity in the mesopic range . Such observations suggests the involvement of a high threshold rod pathway. The early stages of which have been identified in several mammals, including humans. The major goal of the proposed studies to is to determine how the retina contributes to the entrainment of circadian rhythms. The specific aims are to use an in vitro cat retina preparation 1) to demonstrate the form, distribution densities, coupling, and dendritic arborization pattern of ganglion cells that project to the SCN and the ventral lateral geniculate nucleus using anatomical tracers, 2) map the spectral, spatial, and temporal properties, and sensitivity and dynamic range of identified ganglion cells and 3) trace intra-retinal circuits back from the SCN projecting gangling cells to the photoreceptor input using electron microscopy.